Dry plant-tissue consists of about 50 per cent, of carbon, 44 per cent, of oxygen, with a little nitrogen, and 6 per cent. of hydrogen. In a peat-bog, we find the upper part of the scale represented above very well shown: plants are growing on the surface with the normal composition of cellulose. The first stratum of peat consists of browned and partially decomposed plant-tissue, which is found to have lost perhaps 20 per cent. of the components of wood, and to have acquired an increasing percentage of carbon. As we descend in the peat, it becomes more homogeneous and darker until at the bottom of the marsh ten or twenty feet from the surface, we have a black, carbonaceous paste, which, when dried, resembles some varieties of coal, and approaches them in composition. It has lost half the substance of the original plant, and shows a marked increase in the relative proportion of carbon.
Each inch in vertical thickness of the peat-bog represents a phase in the progressive change from wood-tissue to lignite, using this term with its common signification to indicate, not necessarily carbonized ligneous tissue, but plant-tissue that belongs to a past though modern geological age--i.e., Tertiary, Cretaceous, Jurassic, or Triassic. These lignites or modern coals are only peat beds which have been buried for a longer or shorter time under clay, sand, or solidified rock, and have progressed farther or less far on the road to coal. As with peats, so with lignites, we find that at different geological levels they exhibit different stages of this distillation--the Tertiary lignites being usually distinguished without difficulty by the presence of a larger quantity of combined water and oxygen, and a less quantity of carbon, than the Cretaceous coals, and these in turn differ in the same respects from the Triassic.
All the coals of the Tertiary and Mesozoic ages are grouped under one name; but it is evident that they are as different from each other as the new and spongy from the old and well-rotted peat in the peat-bog.
By mere convention, we call the peat which accumulated in the Carboniferous age by the name of bituminous coal; and an examination of the Carboniferous strata in different countries has shown that the peat-beds formed in the Carboniferous age, though varying somewhat, like others, with the kind of vegetation from which they were derived, have a common character by which they may be distinguished from the more modern coals; containing less water, less oxygen, and more carbon, and usually exhibiting the property of coking, which is rare in coals of later date. Though there is great diversity in the Carboniferous coals, and it would be absurd to express their composition by a single formula, it may be said that, over the whole world, these coals have characteristics, as a group, by which they can be recognized, the result of the slow decomposition of the tissue of plants which lived in the Carboniferous age, and which have, by a broad and general change, approximated to a certain phase in the spontaneous distillation of plant-tissue. An experienced geologist will not fail to refer to their proper horizon a group of coals of Carboniferous age any more than those of the Cretaceous or Tertiary.
Anthracite--In the ages anterior to the Carboniferous, the quantity of land vegetation was apparently not sufficient to form thick and extensive beds of peat; but the remains of plant-tissue are contained in all the older formations, though there only as anthracite or graphite--the last two groups of residual products. Of these we have examples in the beds of graphite in the Laurentian rocks of Canada, and of anthracite of the lower Silurian strata of Upper Church and Kilnaleck, Ireland.
From these facts it is apparent that the carbon series is graded geologically, that is, by the lapse of time during which plant-tissue has been subjected to this natural and spontaneous distillation. But we have better evidence than this of the derivation of one from another of the groups of residual products which have been enumerated. In many localities, the coals and lignites of different ages have been exposed to local influences--such as the outbursts of trap-rock, or the metamorphism of mountain chains--which have hastened the distillation, and out of known earlier groups have produced the last. For example, trap outbursts have converted Tertiary lignites in Alaska into good bituminous coals; on Queen Charlotte's Island, on Anthracite Creek, in southwestern Colorado, and at the Placer Mountains, near Santa Fe, New Mexico, Cretaceous lignites into anthracite; those from Queen Charlotte's Island and southwestern Colorado are as bright, hard, and valuable as any from Pennsylvania. At a little distance from the focus of volcanic action, the Cretaceous coals of southwestern Colorado have been made bituminous and coking, while at the Placer Mountains the same stratum may be seen in its anthracitic and lignitic stages.
A still better series, illustrating the derivation of one form of carbon solids from another, is furnished by the coals of Ohio, Pennsylvania, and Rhode Island. These are of the same age; in Ohio, presenting the normal composition and physical characters of bituminous coals, that is, of plant tissue generally and uniformly descending the scale in the lapse of time from the Carboniferous age to the present. In the mountains of Pennsylvania the same coal beds, somewhat affected by the metamorphism which all the rocks of the Alleghanies have shared, have reached the stage of semi-bituminous coals, where half the volatile constituents have been driven off; again, in the anthracite basins of eastern Pennsylvania, the distillation further effected has formed from these coals anthracite, containing only from three to ten per cent. of volatile matter; while in the focus of metamorphic action, at Newport, Rhode Island, the Carboniferous coals have been changed to graphitic anthracite, that is, are half anthracite and half graphite.
Here, traveling from west to east, a progressive change is noted, similar to that which may be observed in making a vertical section of a peat bog, or in comparing the coals of Tertiary, Mesozoic, and Carboniferous age, only the latter is the continuation and natural sequence of the former series of changes.
In the Laurentian rocks of Canada are large accumulations of carbonaceous matter, all of which is graphite, and that which is universally conceded to be derived from plant-tissue. The oxidation of graphite is artificially difficult, and in nature's laboratory slow; but it is inevitable, as we see in the decomposition of its outcrops and the blanching of exposed surfaces of clouded marbles, where the coloring is graphite. Thus the end is reached, and by observations in the field, the origin and relationship of the different carbon solids derived from organic tissue are demonstrated.
It only remains to be said, in regard to them, that all the changes enumerated may be imitated artificially, and that the stages of decomposition which we have designated by the names graphite, anthracite, coal, lignite, are not necessary results of the decomposition of plant-tissue. A fallen tree may slowly consume away, and all its carbonaceous matter may be oxidized and dissipated without exhibiting the phases of lignite, coal, etc.; and lignite and coal, when exposed to air and moisture, are burned away to ashes in the same manner, simply because in these cases complete oxidation of the carbon takes place, particle by particle, and the mass is not affected as a whole in such a way as to assume the intermediate stages referred to. Chemical analysis, however, proves that the process is essentially the same, although the physical results are different.